Parkinson's disease (PD) is a common age-related neurodegenerative disorder in which multiple aspects of visuospatial cognitive function are impaired, including spatial navigation. Deficits in spatial navigation arise from pathological changes in high-order association areas of the brain but also from defective input from lower-level visual processing areas, including those that mediate optic flow. Optic flow refers to the radial visual patterns that indicate a person's direction of self-movement and maintain gait and postural integrity. The status of optic flow perception in PD is unknown, as is its relation to PD deficits in spatial navigation, such as the inability to maintain a straight path while walking. We propose to examine optic flow perception in PD and its relation to spatial navigation. A critical focus is on the side of onset of motor impairment, contralateral to the hemisphere with predominant basal ganglia dysfunction. PD usually has unilateral onset, and many visuospatial abnormalities arise from right-hemisphere dysfunction. Our preliminary studies suggest that PD patients with left motor onset experience spatial compression of the left visual hemifield, which affects their ability to understand spatial relations and to use spatial information for navigation. We predict that deficits in optic flow perception underlie this perceived spatial compression. Specifically, hemifield differences in optic flow velocities lead patients to misperceive a straight path as curved toward the compressed side of space, and they "correct" their trajectory by walking a curved path. We propose to investigate optic flow and spatial navigation in 18 patients with left-onset PD, 18 with right-onset PD, 18 healthy elderly adults, and 18 healthy young adult adults. Our specific aims are: (1) To manipulate the speed of optic flow in the two visual hemifields and measure each participant's perception of relative speed, which is associated with perceived direction of self-motion. (2) To relate optic flow to spatial navigation. We will use a head-mounted system that provides virtual visual input and records the orientation and spatial position of the participant while walking through veridical space. (3) To relate optic flow and spatial navigation to daily function, as assessed with questionnaires. Our project is conceptually innovative in proposing optic flow deficiencies as a cause of problems of spatial navigation in PD and will generate pilot data in this potentially important area. We have forged novel collaborations between experts in visual psychophysics, behavioral neuroscience, biomedical engineering, and physical therapy to accomplish the goals of this project.